1. Field of the Invention
The present invention relates in general to a process for growth of nitride based materials on patterned substrates formed from SiC, sapphire and other materials.
2. Description of the Background Art
Since the report of improved performance and reliability of GaN-based laser diodes grown on epitaxial lateral overgrown (ELO) GaN on sapphire, there has been considerable interest in the deposition process and characterization of nitride materials grown in this fashion. It has been established that dislocation free GaN materials, within the observation limits of transmission electron microscopy (TEM), can be obtained in severely mismatched systems in regions where lateral overgrowth occurs. In these studies, SiO2 masks are patterned on planar heteroepitaxial GaN growth surfaces formed by the deposition of relatively standard nucleation layers on either SiC or sapphire substrates. A multiple growth temperature Organometallic Vapor Phase Epitaxial (OMVPE) process is used regardless of the choice of substrate. Once the planar GaN surface is realized after an initial growth step, the SiO2 mask is deposited and patterned, and the substrate is returned to the OMVPE reactor for a second regrowth step.
A drawback to this process is the need to remove the substrate from the reactor chamber during the mask deposition and patterning process, and then return the substrate to the chamber for the subsequent regrowth step. However, these steps cannot be eliminated because the GaN will grow on the masking material if attempted directly on a patterned substrate in a single step. This is due to the commonly accepted nucleation schemes employed, which initiates GaN-based crystal growth at temperatures in the range of 400-600xc2x0 C. This approach deposits an amorphous film on both the masking material and the window opening that requires re-crystallization at higher temperatures, typically above 1000xc2x0 C. This conformal coating does not allow lateral overgrowth to take place and is therefore undesirable.
The present invention overcomes the drawbacks to the regrowth technique through provision of a single nucleation step process for growth of ELO materials on lattice mismatched substrates. The regrowth step is avoided by initiating the growth process on substrates that have a mask layer formed directly on them, rather than on an initial growth layer. The masked substrates are exposed to an ELO growth process wherein a first layer of nitride based material is grown in the mask openings, thereby forming nucleation sites on the substrate. Then, either continued growth of the layer, or growth of an additional layer of nitride based material material occurs until a planar surface is formed.
In a preferred embodiment, an SiC, sapphire or other suitable material substrate is first patterned with an mask layer, preferably formed of Si3N4. The Si3N4 mask material is preferred for this process because AlGaN used in a subsequent nucleation step is less likely, through thermodynamic arguments, to grow on the masked surface than the more commonly used SiO2 mask. The patterning forms rows of apertures or window openings for subsequent growth of nitride based materials, including GaN, InN, AlN or their related alloys, until a planar film is achieved.
The ELO growth process is preferably carried out by any conventional epitaxial growth technique in a reactor chamber at a temperature of 700-1100xc2x0 C. The high temperature is important to insure that initial material growth will only occur in the mask openings. First, the nucleation step is carried out by growing a layer of nitride based material preferably AlGaN, in the mask openings until lateral overgrowth occurs such that a mushroom shaped cap is formed above each of the openings that overlaps the mask. The growth process is then continued, preferably by forming a layer of GaN or other nitride based material that covers the AlGaN caps, and gradually spreads up and out (growing laterally) until the adjacent rows of growth merge into a single layer. Eventually, continued growth of the GaN layer results in formation of a planar GaN surface that is now suitable for use as a substrate for GaN based devices.